The invention relates to a line driver, more particularly to a line driver with adaptive output impedance. Such a line driver is known from International Patent Application published under No. WO 95/02931. A line driver is an electronic buffer amplifier designed to have an output impedance matched to the characteristic impedance of a transmission line. Transmission lines are widely used for conveying electric signals. To minimise reflections the source and load impedances of the transmission line should be equal to the characteristic impedance of the transmission line. A standard value for video applications is 75 ohms. A buffer amplifier designed to drive a 75 ohm transmission line should have an output impedance of 75 ohms in order to minimize reflections. The buffer sees a load resistance of 75 ohms, i.e. the impedance of the terminated transmission line.
FIG. 1 shows a first known approach to implement such a buffer. The buffer provides a low-impedance voltage source VS with a series resistor Rs having a value equal to the characteristic impedance of the transmission line TL in order to implement the correct output impedance of the line driver. This type of line driver is often used in digital-to-analog converters (DAC) where the output voltage is controlled by a digital input signal. An disadvantage of this first known structure is that the voltage source VS has to deliver twice the desired output voltage. This becomes a problem where the available supply voltage drops while the signal levels remain unchanged; e.g. a 1.5Vpp output voltage at 3V supply voltage is barely possible.
FIG. 2 shows a second known approach. The voltage source with series resistance is replaced by a current source CS with a parallel resistance Rs. In this structure no voltage is lost, but half of the current delivered by the current source CS is wasted in the parallel resistance Rs. This technique is often used in DACs where the output current is digitally controlled.
It appears that both known solutions are not attractive since either voltage or current is wasted. Therefore it can be appreciated that a line driver with specified output impedance which matches the impedance of a transmission line and which does not require twice the output voltage or twice the output current is desirable.